1. Field of the Invention
The invention concerns a monitoring system and method for determining the amount of a material that is emitted from a particular source. More specifically, the invention is directed to a Continuous Emission Monitoring (CEM) system based on a sampling sub-system with a dilution-type extractive probe used in a fossil-fired power plant or in an industrial facility to determine the concentrations of gaseous species emitted from the stack or chimney of the facility. The invention also is directed to analyzer-based process monitoring systems based on a sampling sub-system with a dilution-type extractive probe to measure the concentration of gaseous species in process gases for the purpose of process monitoring, control, and quality assurance.
2. Discussion of the Related Art
The Environmental Protection Agency (EPA) requires power generating plants and other industrial facilities to report pollutant emissions and, based upon reported emissions, sets up a market-based trading system for establishing emissions allowances. Accordingly, it is advantageous for industries to record emissions from its plants as accurately as possible. Conventionally, a CEM system is used to analyze and correct data received from a probe located in or adjacent to a stack or ducts to determine the contents of gas that is emitted from the plant.
Industrial plants commonly use a CEM system with xe2x80x9cdilution-type sampling.xe2x80x9d Such a CEM system uses a dilution probe, which is inserted into the plant""s stack or ducts to obtain sample emissions of the flue gas. The sampled gas containing pollutant and/or other combustion by-products is typically referred to as flue gas, sample stack gas or emission gas and can also be considered emitted material. The dilution probe can be located anywhere in the ductwork, air pollution equipment or stack where a representative volume of flue gas can be obtained. The dilution probe of the CEM system includes an inlet for the sample gas, two input gas lines (dilution gas line and calibration gas line) and one output gas line (diluted sample gas line). In operation, a dilution gas, which can comprise clean dry dilution air, is injected into a venturi device causing flue gas to be drawn into the venturi through a sonic (critical) orifice. The flue gas is mixed with dilution air to create mixed dilution sample gas. The mixed dilution sample gas is then delivered to an analyzer via the diluted sample gas line, and the analyzer determines the concentration of emitted pollutants in the mixed diluted sample gas. A particulate filter is often included in the dilution probe to remove particulate matter from the sample gas upstream of the sonic orifice to prevent plugging.
In order to consistently measure the contents of stack emissions, a dilution ratio is determined and used to manipulate the data output by the analyzer. Specifically, the dilution ratio is multiplied by the results from the analyzer to compensate (or correct) for the effect of dilution air in the flue gas being analyzed. The result is an indication of the concentration of the different materials, e.g., pollutants, being emitted in the flue gas.
Typically, the dilution ratio has been defined by the following formula based on inferred volumetric measurements of dilution and flue gases:
D=(Qdil+Qstack)/Qstackxe2x80x83xe2x80x83(1) 
where D is the dilution ratio, Qstack is the volumetric flow of flue gas and Qdil is the volumetric flow of dilution air.
The data obtained from the analyzer are converted to determine the concentrations of various gaseous species being discharged from the stack. This is done by multiplying the analyzer readings by the dilution ratio evaluated at the time of calibration, as follows:
Csack=Canalyzerxc2x7Dxe2x80x83xe2x80x83(2) 
However, several variables affect the calculation of the dilution ratio. These variables include the use of unheated dilution probes, temperature differential errors that occur when using a heated dilution probe, variability of stack and barometric pressure, inadequate dilution air pressure regulation, inadequate internal heat exchanger performance, variability of temperature in the sampling system transport lines or umbilical, variability of molecular weight of the calibration, reference, and flue gases, variability of calibration gas flow rate induced temperature effects on dilution air mass flow rate, variability of calibration gas flow rate induced pressure effects on sonic orifice inlet (stagnation) pressure, dilution probe temperature, sample temperature, and dilution air supply pressure. Thus, the dilution ratio typically used in CEM systems can be inaccurate and this results in errors in reported emissions or process data.
A CEM system must also be calibrated periodically to ensure accurate readings. In order to calibrate a CEM system dilution probe, calibration or reference gases are injected into the probe. The calibration verifies that the system errors at zero, low, mid and high concentration levels are within prescribed acceptable limits. These calibration or reference gases can also be referred to as span gases.
One example of an attempt to account for variations in the dilution ratio is disclosed in U.S. Pat. No. 5,596,154 to Baughman (the ""154 patent). The ""154 patent discloses a dilution control apparatus for use in a CEM system which includes a regulator for regulating the flow of dilution gas to a dilution probe, a mechanism for measuring changes in gas density of the emission gas and a mechanism for determining an adjusted flow rate of the dilution gas based on the measured changes in gas density to control the dilution ratio. The xe2x80x9cgas densityxe2x80x9d measured by the device disclosed in the ""154 patent is a measure of change in stack and atmospheric pressures (see column 4 lines 38-40 of the ""154 patent specification). In operation, the device disclosed in the ""154 patent monitors the stack and atmospheric pressures and, upon a change in xe2x80x9cgas densityxe2x80x9d, determines an adjusted flow rate of dilution gas and regulates the flow rate of dilution gas to the adjusted flow rate. Accordingly, the dilution ratio is maintained constant during the sampling process and less data manipulation is necessary to determine the constituents of the stack gas from the stored analyzer data.
Several problems exist in the conventional systems for correcting the dilution ratio such as disclosed in the ""154 patent. Specifically, the calculation of the material emitted from the stack can be inaccurate due to the fluctuation of variables during sampling of the stack gas. These variables include dilution probe temperature, sample absolute pressure, sample temperature, dilution air supply pressure, calibration gas molecular weight, sample gas molecular weight, and umbilical cable temperatures. In addition, physical control of various stack emission and data gathering parameters can be expensive and difficult to operate and can further intensify the possibility of error in calculation of the material constituents of the stack emission.
In light of the above points, an object of the invention is to provide a CEM system and method that more accurately measures material and/or pollutants emitted from a source (e.g., a stack). The invention includes a CEM system that utilizes a dilution ratio based on molar flow rates of dilution air and flue gases in order to obtain a more accurate dilution ratio. The invention enables a more consistent and precise measurement of the concentrations of gaseous species emitted from a source without requiring additional mechanisms for controlling parameters of flue gas flow. The formula for dilution ratio based on molar flow rates is as follows:
D=({dot over (n)}dil+{dot over (n)}stack)/{dot over (n)}stackxe2x80x83xe2x80x83(3) 
where D is the dilution ratio, {dot over (n)}dil is the molar flow rate of dilution air, and {dot over (n)}stack is the molar flow rate of flue gas.
The molar flow rates of the gases are calculated using a functional equation (algorithm) based on various data measured at the stack, such as the temperature and pressure of the flue gas. Thus, not only does the dilution ratio (3) correct the data produced by the analyzer for error due to the presence of dilution air, but it also eliminates error due to different variables including pressures (dilution air supply pressure, barometric pressure, critical orifice stagnation pressure), temperatures (including dilution air supply temperature, stack or duct temperature, critical orifice stagnation temperature), calibration gas molecular weight, and sample molecular weight that affect the ratio between dilution air and flue gases. In addition, the invention improves gas measurement accuracy by reduction or elimination of uncertainty due to the following: use of unheated dilution probes; temperature differential errors that occur when using a heated dilution probe; variability of stack and barometric pressure; inadequate dilution air pressure regulation; inadequate internal heat exchanger performance; variability of temperature in the sampling system transport lines or umbilical; variability of molecular weight of the calibration, reference, and sample gases; variability of calibration gas flow rate induced temperature effects on dilution air mass flow rate; variability of calibration gas flow rate induced pressure effects on sonic orifice inlet (stagnation) pressure. Sampled gas molecular weight can also be corrected to constant test values or can be dynamically compensated if desired.
Additional features and advantages of the invention will be set forth in the description that follows, and in part, will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and achieved by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve this and other advantages, and in accordance with the purpose of the invention, as embodied and broadly described the present invention provides for a flow measurement and analysis device to be used in conjunction with a dilution probe. The combined system is capable of providing accurate measurement of the molar flow rate of sample gas and dilution air, which together flow to the gas analyzers for subsequent analysis of flue gas composition. In accordance with another aspect of the invention, a system for use with an emission source includes a data medium for receiving emissions data collected from sensors located at the emission source, the emissions data including an amount of a particular material emitted from the emission source, and means for calculating a total amount of the particular material emitted from the emission source using a mass flow rate value.
In accordance with yet another aspect of the invention, a method for calculating an amount of a material emitted from a source includes sensing an emission gas emitted from the source; determining a mass flow rate value of one of the emission gas and a dilution air; calculating a dilution ratio based on the determined mass flow rate value; and determining a characteristic of the emission gas using said dilution ratio.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.